Supermarket security staff on the look-out for shoplifters one day could be guarding Britain’s nuclear deterrent the next, Unite, the country’s largest union, has warned.

The union is concerned that the proposed privatisation of the Ministry of Defence Guard Service (MGS) will lead to a drop in standards and training with the possible adverse repercussions for the security of such sites as the nuclear bases at Faslane and Coulport in Scotland.

The MGS employs more than 2,000 guards providing unarmed security for more than 100 Ministry of Defence (MoD) sites, including the MoD main building in Whitehall and the naval bases at Portsmouth and Devonport.

Unite national officer for the MoD and government departments Mike McCartney said: “The UK government is planning to open up the MGS, whose personnel are well-trained, to privatisation.

“Unite wholeheartedly opposes these plans and considers any such proposals are irrational and dangerous to the security not only of MoD sites, but to the security of our service personnel, their families and to all the civilians employed across these sites.

“If these proposals went ahead we could see someone deterring shoplifters at a supermarket on a Sunday then that same person turning up to guard a nuclear site on Monday. Employment conditions, training and standards could be compromised in ‘a race to the bottom’ by a private company keen to maximise profit.

“The prospect of an outsourcing company such as G4S, which was in charge during December’s prison riot in Birmingham and responsible for the security fiasco at the 2012 Olympics, taking over the MGS makes the heart sink and should be a matter of serious public concern.

“The MoD believes that by privatising the MGS there could be a saving about £17 million over five years, however, Unite does not believe that these savings could be made.

“We further believe that this is not a cost saving exercise, but merely a ruse so the MoD can meet the government’s target to axe 30 per cent of the civilian workforce across the whole of the MoD. These controversial plans should be dropped immediately.”

The union said that private security companies would be required to bid on a criteria which is currently based on 70 per cent on price and only 30 per cent on quality.

Unite has met with Nia Griffiths, the shadow defence secretary, who has now tabled a question on this issue to the defence secretary Sir Michael Fallon.

Unite has also made contact with Jackie Baillie, Labour MSP, whose Dumbarton constituency includes Faslane, who has also written to Fallon. She has laid down a motion at the Scottish Parliament calling for an official debate, acknowledging the vital role the MGS plays.